We propose to study the developmental changes in two key properties of mammalian myocardium -- the force-interval relationship and the dynamic properties of the cardiac sarcomere. We will use sheep hearts at a variety of ages from the fetus to the adult. The normal development will provide the control for analyzing the changes in these properties that accompany hypertrophy. Cardiovascular lesions will be produced in the in utero lamb. The time-course of developmental changes in the force-interval relationship and sarcomere dynamics that accompany the resulting hypertrophy of the heart will be determined and compared to the normal developmental changes. We anticipate that the results of this study will provide a basis for understanding myocardial performance, both in the adult and in the infant heart and will form an integral part of the analysis of the results of our ongoing experiments on the chronically instrumented fetal and newborn lamb. In addition to their obvious relevance to understanding the process of development these results will shed light on the subtle and complex relationship between sarcomere length and muscle tension and the way this relationship is altered by inotropic changes in the muscle and the development of cardiac hypertrophy. These insights should lead to a better understanding of myocardial performance under normal and abnormal work loads and so provide the basis for useful and new diagnostic methods in evaluating not only the infant and the child but also the adult.